Method of regenerating anodizing baths



March 2, 1948. J, SCHULEW 2,436,940-

METHOD OF REGENERATING ANODIZING BATHS Filed Aug. 2, 1946 2 Shets-$heet 1 ANOD/Z/NG BA TH LIME Z FILTER AL UM/NUM CONTAIN/N6 PREC/P/TATE PPECIPITATE a v a co" l 3 ANOD/Z/NG BATH [m me v Bfi $04 PR'C/P/TATE l7 HF 1/ v & F2 ZKQZ PRECIPITATE INVENTOR;

JOSEPH SCHULE/N 4w ATTORNEYS.

March 2, 1948 Filed Aug. 2, 1946 2 Sheets-Sheet 2 M: 014 (To MAKE UP LOSSES) 20 ANOD/Z/NG M; 0H

8A TH 1v, 00

STORAGE 28/ Na. OH

' NG-g Cr 04 21 H2 Cr a, smu. AMOUNT 22 Off/V0. Cr: 07

nus? PEECIPITA TE CONTAINING ALUMINUM 23 STORAGE ANOVL YTE c4 THOL YTE 1 25 I I 27 T0 3 ANOD/ZING ELECTROLYTIC BATH CELL N0H+ SMALL AMOUNT OF WEE Ma; Cr 04.

INVENTOR.

JOSEPH SCHULE/N M M? M J. SCHULEIN 2,436,940

Patented Mar. 2, 1948 UNITED S'TATESNPATENT OFFICE METHOD OF REGENERATIN G AN ODIZIN G BATHS- Joseph Schulein, Corvallis, Oreg.

Application August 2, 1946, Serial No. 688,038

15 Claims. 1

This invention relates to the treatment of chromic acid solutions to remove impurities therefrom and relates more particularly to the regeneration of spent chromic acid anodizing solutions.

This is a continuation-impart of my abandoned applications Serial, No. 494,772, filed July 15, 1943, and Serial No. 495,165, filed July 17, 1943.

Chromic acid anodizing solutions are common- 1y used in the treatment of aluminum and aluminum alloys to render the surfaces of the metal corrosion resistant. Large quantities of aluminum and aluminum alloys are treated by anodizing methods inasmuch as anodizing is a required treatment for the metal used in army and navy matriel.

The anodizing process includes the immersion of the metal in the anodizing solution and the use of the metal as an anode in an electrolytic process. The bath and the electric current convert the surface of the aluminum-containing metal into a mixture of aluminum oxide and aluminum hydroxide which protects the underlying metal because of the relative imperviousness and inertness of the oxide-hydroxide film.

The preferred type of anodizing solution, from the standpoint of optimum film-forming characteristics should contain about chromic acid. A newly prepared chromic acid bath of that concentration will have a pH value of approximately 0.1. The exact pH value is dependent upon the concentration of chromic acid.

During the anodizing treatment, aluminum is dissolved from the surface of the material being treated and enters the bath, where it combines with some of the chromic acid to form aluminum dichromate. This compound is only slightly dissociated and, therefore, has the effect of decreasing the available chromic'acid in the bath. The chromic acid that is combined in an undissociated state with the aluminum is commonly referred to as fixed acid, while the chromic acid that is not combined in such an undisso'ciated state is commonly referred to as free acid.

Aluminum dichromate contains about 92% chromic acid and about 8% aluminum. Therefore, a small amount of aluminum dissolved in the bath causes a considerable decrease in the available free acid for the reason that it comblues with about eleven times its own weight of chromic acid. Thus, for every pound of aluminum dissolved in the bath, 11 pounds of chromic acid are converted from the free state to the fixed state and become unavailable for continued formation of the anodic film.

It is the concentration of free acid that determines the acidity of the bath. Inasmuch as there is a definite relationship between the acidity of the bath and the characteristics and amount of anodic film produced, it is essential that the acidity of the bath be controlled within definite limits.

It has been found that the anodizing bath functions best if the acidity is kept within the range of pH 0.6 to pH 1.3. These pH conditions are commonly maintained by the periodic addition of chromic'acid anhydride in an amount necessary to compensate for the chromic acid being "fixed or combined in an undissociated state with the aluminum' dissolved in the bath. The constant addition of chromic acid to the bath contributes to an ever-increasing concentration of total chromic acid in the bath. Under these conditions, the bath rapidly approaches a condition of extremely high density, and operating difficulties arise because of changes in electrical conductivity, viscosity, etc. Also, losses are encountered for the reason that as the work is removed from the bath, the liquid film adhering to the metal is of higher chromic acid concentration.

In order to compensate for these variations in the condition of the bath, it has been common practice to start a new bath at a low chromic acid concentration, such as 3 to 5% and then add chromic acid to the bath as necessary, thereby prolonging the time until the chromic acid concentration becomes so high that the bath isno longer useful. Baths containing low concentrations of free chromic acid and baths containing a high fixed chromic acid concentration do not produce the best quality film. Therefore, quality of the anodic film has been sacrificed and nonuniformity of the film has resulted from the prior practice.

When the bath attains a concentration of fixed acid such that the use of the bath is no longer practical, the bath is discarded. Many thousands of gallons of spent chromic acid solutions have been discarded annularly because of the above-described conditions.

An object of the present invention is to provide a method of treating chromic acid solutions containing fixed acid to convert the fixed acid into free acid.

Another object of the invention is to provide a method of treating spent chromic acid anodizing solutions in order to recover the chromic acid therein and to permit the bath to be operated indefinitely.

'Another object of the invention isto provide a method of regenerating chromic acid anodizing baths to allow the operation of the bath within a narrow range of pH values and with only a small variation in the amount of fixed and free chromic acid in the bath, to cause the formation of more uniform anodic films.

Other objects of the invention will become apparent from the following description of typical methods of treating chromic acid solutions in accordance with the present invention.

In accordance with the present invention, chromic acid baths or solutions containing dissolved aluminum are treated to cause a conversion of the soluble aluminum dichromate therein into an insoluble aluminum compound and free chromic acid.

More particularly, the process includes the par-- tial neutralization of chromic acid anodizing baths in order to convert the soluble aluminum calcium sulphate.

tion of calcium chromate. In other words, the chromic acid should be about one-half neutralized by the addition of the lime.

After adjustment of the pH value and the formation of the aluminum-containing precipitate, the solution is passed to a filter l2 where the aluminum hydroxide precipitate is separated and washed. The filtrate is delivered to another tank l3 for further treatment.

A suflicient amount of sulphuric acid is added to the filtrate in the tank l3 to convert the calcium dichromate into chromic acid and insoluble The solution may be adjusted to a value between about pH 0.6 and pH 1.3, as

5 desired, by the addition of sulfuric acid. This mixture is passed to the filter l4, thereby removshould not be much below pH 3.5 if substantially complete separation of the aluminum-ions is to be obtained. At a pH value of about 10, or above, some of the aluminum hydroxide will dissolve and moreover the chromic acid will be converted into a chromate. required to neutralize the solution to higher pH values and no advantage is obtained thereby. Therefore, for practical purposes, neutralization of the anodizing bath to'a pH value range of 3.5 to 5.0 is most advantageous.

V The precipitate obtained in the pH value range of 3.5 to 5.0 may be regarded as aluminum hydroxide containing absorbed chromic acid. This precipitate may be filtered from the balance of An excessive amount of alkali is the solutionand washed with water at a maintained pH value higher than 3.5 so as to separate most of the absorbed chromic acid from the aluminum hydroxide as well as promote hydrolysis of any aluminum dichromate that may not have been hydrolyzed.

The filtrate is then treated to restore it to a desired pH value and may be treated further to eliminate unwanted ions introduced by the preceding treatments, whereby a purified bath is obtained with relatively little loss ofchromic acid.

For a better understanding of the reactions involved, reference may be had to the accompanying drawing in which Figure 1 is a flow sheet of a typical process for regenerating chromic acid anodizing baths; and Figure 2 is a fiow sheet of a modified process for regenerating such chromic acid anodizing baths.

In accordance with the present invention, and as illustrated in Figure l, I withdrawa portion of the chromic acid solution from the anodizing bath I0 and deliver itto a tank H where it may be partially neutralized by the addition of calcium hydroxide, calcium carbonate or calcium oxide, or metallic calcium, thereby forming calcium dichromate in solution, plus an aluminum-containing precipitate. The amount of the calcium containing neutralizing agent should be sufiicient to increase the pH value of the solution to about 3 or above, and preferably within a pH value range of 3.5 to 5, thereby converting the chromic acid to calcium dichromate and avoiding the formaing most of the calcium sulphate and recovering a filtrate of chromic acid that may be returned to the anodizing bath ID.

This process may be used for the treatment of an entire bath of the chromic acid solution or it may be used to regenerate portions of the bath so that the bath can be operated continuously within a narrow range of predetermined pH values. For example, after several days operation, a portion of the bath containing an amount of aluminum equivalent to the amount of aluminum being dissolved in the entire bath each day may be removed and regenerated as described above. Thus, periodic treatment of small portions of the bath by the above-described method maintains the bath in constant operating condition.

The calcium sulphate precipitate, previously mentioned, is slightly soluble in the chromic acid solution, and, therefore, the solution which is returned to the anodizing bath contains a small amount of calcium sulphate.

If it is desired to reduce the amount of sulphate present in the anodizing bath, this may successfully be accomplished by treating the bath further before returning it to the bath. Thus, the filtrate from the filter l4 may be delivered to the receptacle l5 where barium carbonate is added to the solution in a quantity sufficient to react with the calcium sulphate to form barium sulphate which is less soluble than calcium sulphate. The calcium ions combine with the chromic acid to form calcium dichromate. The calcium dichromate has no effect on the anodizing bath other than using up a small amount of chromic acid in the fixed state.

The solution may be passed to the filter l6 to separate out the barium sulphate precipitate and the filtrate may then be returned to the anodizing bath.

The bath may be purified further by deliverin the filtrate from the filter Hi to a receptacle I7, where hydrofluoric acid is added in an amount suificient to precipitate the calcium in the form of calcium fluoride. The precipitated calcium fluoride is removed by the filter l8 and the filtrate containing a small amount of dissolved calcium fiuoride is returned to the anodizing bath.

Upon adding the latter mixture to the anodizing bath, the calcium fluoride immediately reacts with the aluminum dichromate present in the bath to form calcium dichromate and aluminum fluoride, and undissociated compound which has no effect upon the operation of the anodizing bath.

While the process has been disclosed in the drawing in Figure 1 as a single fiow sheet, it will be understood that the process may be practiced in part, or as a whole, as described.

By wayof typical example of the mucosa a sample was ordained the bat-h used in the treatment of aluminum The total :chromic ,present'ms eqluyalent to .112 grams per liter .of 0103 or .98 grams per liter was flfixe'd CrOa mmhined'with' alumiand the remainder; l4 or mi was free chromic acid. This sample was neutra iized $0 9, value 'fif $.27 calcium hydroxide to cause precipitation "of the aluminum ozs allummum hydroxide. nitration to separate the aluminum hydroxide precipitate, the .filtm'te was adjusted to a pi-Iyalue of animus with 3u'l-' l 'huric acid :and ifi'ltened to remove 'icailcrum sulpha'te. Analysis of the filimate showed that 98.3% .of the fixed and me ohromrciacidwas recovered as tree 'chromic acid. dflui fiher treatment to remove sulphate and calcium ions by the addition of barium carbonate and hydrofluoric acid in the manner described above and :in amounts chemically equivalent to the omwurrt of calcium ions present, did not cause substantial toss of the .free chromic acid. 7

While the process been descrlbedwith reference to the @use of calcium hydroxide or .lime as the neutralizin agent for the solution; it will he understood that :neutraiization rand .aprecipitation can be practiced with metallic calcium or calcium carbonate. The operations would zbe the same if these other materials were used in the proper equivalents or chemically :stoictnometrio epropo-rtions. Moreover, one or more (of the steps of the process can :be combined, if desired. For ple, barium :fiuoride may :be used instead of barium carbonate to the of sulphate present, and to combine the zptec'ipltation of barium sulphate and calcium lluouide in one step.

The invention can be practiced other ways also, thus, as described hereinafter a. spent or contaminated anodizing loath-can he treated to remove zalum-inum ions and the-matter treated electrolytically to remove rim-wanted metallic "ions.

In accordance with the present invention and as illustrated in Figure 2, 1 withdraw a or all of the chromic acid solution containing aluminum dichromaite iron: the anodizing bath 520 and deliver to fa tank 21 where it fs partially neutralized by the addition "of an alkaline whose metallic ion Forms compounds that are soluble in the solution, such as, for example, sodium or potassium hydroxide, lime, calcium hydroxide or calciumcarbonate. Sodium hydroxide is used, preferably, and the process be described with reference to the use of sodium hydroxide as the alkaline agent. The addition of sodium hydroxide to the chromic "acid solution results in the formation or sodium *dichromate in solution, plus an aluminum-containing precipitate. 7 other alkaline agent be sufficient to increase the pH value of the solution to about 3 or above and preferably a "value range of 3.5 to 5, thereby converting the 'dlrromic acid to sodium d ichromate and avoidingthe torm ai tion of sodium chromate. In other words, the :chromic acid should be about one-halt neutralized by the addition of the alkaline agent.

After adjustment of pH value and the formation of an aluminum-containing precipitate, the solution is passed to 'afi'lt'er 2 2, where the aluminum hydroxide precipitate is separated out and washed alkaline water to remove any sodium dichromate remainingin the irecipi t-ate.

Ihe amount of sodium hydro'ititha or of thesdiaphragm.

, oI-aims. V

flln'szte is delivered to-storage tank 223 and thence to .o; diaphmgm *of electrolytic cell 24 an anode 25 on one side of the :dia-

Zfivanda cathode 2:1 onthe other side When direct current is passed through the cell, the :didhromateions will be segregated as chromic or adichromic acid in the anolyte, and the sodium ions will -rb'e segregated in the catholyte, forming sodium hydroxide therein. The current andvoltage used during electrolysis "can be varied considerablyflbut' preferably a current density and a voltage :are used -=that will-not :cause excessive discharge of otheryions, particularly hydrogen and oxygen, amounts sufficient to eflectively reduce the overall efiiciency below :a predeten' mined, economic minimum taking into account the discharge :potentials of these ionson various electrodes thatimay he used and the economic balancing {of 9061'] size and capacity at any given current density :agai-nst initial and operating costs.

The chromic acid in the a-nolyt-e is returned to the anodizing bath. The sodium hydroxide solution is separated and :is returned to -a storage tank 2B :for use in the neutralization of the next batch of anodizing solution,

V A perfect separation of the sodium ions from the :chrcmie acid solution or a complete separationwof ithechromic acid from the sodium hydroxide is not necessary as these solutions merely go around and around the circuit. The separation should beisumciently ioomple'te, however, tor'mainrain the anodizing bath within a pH value range of about 0. 1 to 1 3. V

.As -:a. specific example of the-above process, :a portion of an :amodizing bath solution was neutraiized to a pl-I vadue of =4;2'with sodium hydroxide rand filtered. The filtrate, contained 101,3 grams/liter CrQa in the form of sodium ,diohromate. The filtrate was fed to an electrolytic cell. The electrolysis was conducted W'itha current of '015 ampere and a, voltage :of 1 5 to '18 was used to hold the current :con'stan't. After electrolysis; the an'oly-te liquor contained 95 grams/liter of free C lOs and had :a pH value of. 0.13.

"Iihe recovery rate was "8:7 pounds :of CrOa per thousand ampere hours, which at a 3-volt cell dropis' equivalent to 2.89 pounds of CrOs per kilowatthour. i

the preceding description, it will be clear that I have provided simple, yet effective methods =of regenerating anodizing baths, whereby siuastantial economies are obtained in the anodizin-g operation and the prevention of waste :of a vital moteflozl. It will be understood that the processes can he varied considerably without depanting from the invention, for example, all or a nart' of the bath may be treated periodically, the current densities used during electrolysis in accordance with the method'illustrated in Figure 2 of the drawings canioe varied and the pH value of theregen'erated'loath can be controlled as the impose demands. "-Iherefc'me, the :aibove-describes. 'meumos should be construed as illustrative and not as limiting the scope oi the followln'g Iclai-m: 1. A method of regener'auaing a, chromic acid anodizing solution having a pH value less than 3 and containing undissoc'iated 'dichromate, which comprises adding an alkaline agent of the class consisting of sodium "hydroxide, po-

tassium hydroxide, lime, 'cal'ciumjhydroiiid e and to calcium carbonate to said solution-to increase its pH. value to not less than 3 and form an aluminum hydroxide precipitate, separating said precipitate from said solution, and thereafter removing the metal ions of said alkaline agent to adjust the pH value of said solution to, less than 1.3.

2. A method of regenerating a chromic acid anodizing solution having a pH value less than 3 and containing undissociated aluminum dichromate, which comprises adding lime to said solution to increase its pH value to between about 3.5 and 5.0 and form an aluminum hydroxide precipitate, separating said precipitate from said solution, and thereafter removing the metal ions of said alkaline agent to adjust the pH value of said solution to a pH value of less than 1.3.

3. A method of regulating the pH value of a chromic acid anodizing bath containing aluminum dichromate which comprises withdrawing a portion of said bath, adjusting the pH value of the portion to not less than 3 with sodium hydroxide to form an aluminum hydroxide precipitate, removing said precipitate from said portion, thereafter electrolyzing said portion in a diaphragm cell to form an anolyte having a. pH value between about 0.1 and 1.3 and mixing said anolyte with the remainder of said bath.

4. A method of regulating the pH value of a chromic acid anodizing bath containing aluminum dichromate which comprises withdrawing a portion of said bath, adjusting the pH value of said portion to between about 3.5 and 5.0 with an alkaline agent of the classconsisting of sodium hydroxide, potassium hydroxide, lime, calcium hydroxide and calcium carbonate, removing from said portion any aluminum-containing precipitate formed therein, thereafter electrolyzing said portion in a diaphragm cell to form an anolyte having a pH value between about 0.1'and 1.3 and mixing said anolyte with the remainder of said bath.

5. A method of regulating the pH value of a chromic acid anodizing bath containing aluminum dichromate which comprises withdrawing a portion of said bath, adjusting the pH value of said portion to between about 3.5 and 5.0 with sodium hydroxide, removing from said portion any aluminum-containing precipitate formed therein, thereafter electrolyzing said portion in a diaphragm cell to form an anolyte having a pH value between about 0.1 and 1.3 and mixing said anolyte with the remainder of said bath.

6. A method of maintainin within an optimum operating range the pH value of a chromic acid bath for anodizing aluminum-base metals and which contains undissociated, soluble aluminum dichromate, which comprises withdrawing a portion of said bath when said bath attainsa pH value of about 1.3, adjusting the pH value of said portion to not less than 3 with an alkaline agent of the class consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, lime and calcium carbonate, to form a precipitate of aluminum hydroxide, separating said precipitate from said portion, thereafter electrolyzing said portion in a diaphragm cell to form an anolyte having a pH value of less than 1.3, and mixing said anolyte with the remainder of said bath.

'7. A method of maintaining the pH value of a chromic acid bath for anodizing aluminumbase metals within an optimum operating range of pH values not exceeding about 1.3, said bath containing undissociated aluminum dichromate, which comprises withdrawing a portion of said bath when said bath attains a pH value of about 1.3, adjusting the pH value' of said portion with sodium hydroxide to not less than 3, to form a precipitate of aluminum hydroxide, separating said precipitate from said portion, thereafter electrolyzing said portion in a diaphragm cell to form an anolyte having a pH value of less than 1.3, and mixing said anolyte with the remainder of said bath.

8. A methodof maintaining the pH value-of a chromic acid bath for anodizing aluminum-base metals within an optimum operating range of about 0.6 to 1.3, said bath containing soluble aluminum dichromate, which comprises withdrawing at least a portion of said bath when said bath attains a pH value as high as the highest pH'value set for the optimum operating range, adjusting the pH value of said portion to not less than 3 with an alkaline agent of the class consisting of sodium, potassium and calcium hydroxides, lime and calcium carbonate, to form a. precipitate of aluminum hydroxide, separating said precipitate from said portion, thereafter electrolyzing said portion to form an anolyte having a pH value within said optimum operating range, and mixing said anolyte with the remainder of said bath.

- of said bath.

10. A method of controlling the pH value of a, chromic acid bath for anodizing aluminumcontaining articles, comprising segregating a portion of said bath, adding lime to said portion to adjust the pH value of said portion to at least 3.0 and precipitate an aluminum compound in said portion, filtering said portion to remove the insoluble aluminum compound, adding sulfuric acid to the filtrate to adjust its pH value to between about 0.6 and 1.3 removing an precipitate formed thereby, adding a soluble barium compound to said filtrate, removing any precipitate formed thereby, and adding the filtrate to the remainder of said bath.

11. A method of controlling the pH value of a chromic acid bath for anodizing aluminumcontaining articles, comprising segregating a portion of said bath, corresponding substantially to the amount of aluminum dissolved in said bath in a predetermined period, adding lime to said portion to adjust the pH value of said portion to at least 3.0 and precipitate an aluminum compound in said portion, filtering said portion 'to remove the insoluble aluminum compound, adding sulfuric acid to the filtrate to adjust its pH value to between about 0.6 and 1.3, removing any precipitate formed thereby, and adding the filtrate to the remainder of said bath.

12. A process for regenerating a chromic acid solution from an anodizing bath for aluminum and aluminum base 9.1loys comprising adding lime to adjust the acidity of the solution to a pH'va-lue greater than 3.0 to form a. precipitate, removing said precipitate, and adding sulphuric acid to said solution to reduce its pH value to not more than about 1.3.

13. A process for regenerating a chromic acid solution from an anodizing bath for aluminumcontaining articles comprising adding lime to said solution to adjust its acidity to a pH value greater than 3.0, filtering said solution, readjusting the acidity of the filtrate by the addition of sulphuric acid, filtering the adjusted solution, treating the filtrate from the adjusted solution with barium carbonate, removing any precipitate formed thereby, adding hydrofluoric acid to the resulting solution and removing the precipitate formed thereby.

14. A process for regenerating a chromic acid solution from an anodizing bath for aluminum containing articles, comprising adding lime to said solution to adjust its acidity to a pH value greater than 3.0, removing any precipitate formed thereby, adding sulphuric acid to the solution to reduce its pH value to less than 1.3, removing any further precipitate formed thereby, and adding barium fluoride to said solution. 7

15. A process for regenerating a chromic acid solution from an anodizing bath for aluminumcontaining articles, comprising adding lime to the solution to adjust its acidity to a pH value between about 3.5 and 5.0 to' form a precipitate, removing 25 said precipitate, adding sulphuric acid t the solution to separate calcium ions as calcium sulfate, removing said calcium sulfate, adding barium fluoride and removing any precipitate formed thereby.

JOSEPH SCHULEIN'.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Monthly Review of the American Electroplaters Society, Oct. 1942, pages 831 through 837.

Hackhs Chemical Dictionary, 2d edition (1937), page 543 (Lime). 

